Support is requested for biochemical studies on recombination in yeast particularly on the role that nucleases play in this process. Recent evidence supports a model for recombination in yeast in which DNA containing a double strand break is an intermediate. We have identified a nuclease which cleaves double stranded DNA at the mating type locus to initiate mating type interconversion and a second double strand endonuclease whose function is unknown. The specific aims of this proposal are to continue our characterization of these two enzymes and to ascertain their role in recombination, to identify other nucleases present in cells undergoing meiosis and determine their function in recombination, and to identify other components of the eucaryotic recombination system. Our long term goal is to identify the various components required for eucaryotic recombination and to begin to understand the behaviour of chromosomes during the complex process of meiosis.